a variant of amid (q.v.) with adverbial genitive -s and parasitic -t. Amidde became amyddes (13c.) and acquired a -t by 1560s, probably by association with superlatives in -st; the pattern also yielded amongst, against, betwixt, whilst.

There is a tendency to use amidst more distributively than amid, e.g. of things scattered about, or a thing moving, in the midst of others. [OED]